[JL8Jeff]

If you have an aftermarket cat that is a true 3" inlet/outlet, then it's not costing you any real power at all(maybe 2-3 hp). The stock cat-back and muffler will cost 10-15 hp easily since it necks down to 2-1/2". But if you have an aftermarket cat-back then it's probably a real 3" cat-back and not costing much power at all. The Loudmouth sounds like crap on an LT1 car but it will make the most power since there is no restriction. The aftermarket cat-back exhausts don't weigh as much as the factory because they are thinner metal. Are you looking for the loudest, most obnoxious exhaust or are you looking for the best power combination? You don't want to run !cat with a Loudmouth.